1. Field of the Invention
The present invention relates to a wireless power supply technique.
2. Description of the Related Art
In recent years, wireless (contactless) power transmission has been receiving attention as a power supply technique for electronic devices such as cellular phone terminals, laptop computers, etc., or for electric vehicles. Wireless power transmission can be classified into three principal methods using an electromagnetic induction, an electromagnetic wave reception, and an electric field/magnetic field resonance.
The electromagnetic induction method is employed to supply electric power at a short range (several cm or less), which enables electric power of several hundred watts to be transmitted in a band that is equal to or lower than several hundred kHz. The power use efficiency thereof is on the order of 60% to 98%. In a case in which electric power is to be supplied over a relatively long range of several meters or more, the electromagnetic wave reception method is employed. The electromagnetic wave reception method allows electric power of several watts or less to be transmitted in a band between medium waves and microwaves. However, the power use efficiency thereof is small. The electric field/magnetic field resonance method has been receiving attention as a method for supplying electric power with relatively high efficiency at a middle range on the order of several meters (A. Karalis, J. D. Joannopoulos, M. Soljacic, “Efficient wireless non-radiative mid-range energy transfer” ANNALS of PHYSICS Vol. 323, January 2008, pp. 34-48)
FIG. 1 is a diagram showing a wireless power transmission system according to a comparison technique. The wireless power transmission system 1r includes a wireless power supply apparatus 2r and a wireless power receiving apparatus 4r. The wireless power supply apparatus 2r includes a transmission coil LTX, a resonance capacitor CTX, and an AC power supply 10r. The wireless power receiving apparatus 4r includes a reception coil LRX, a resonance capacitor CRX, and a load 70.
With such a wireless power transmission system 1r, in order to provide high-efficiency electric power transmission, there is a need to satisfy the conditions for resonance in the entire system including the wireless power supply apparatus 2r and the wireless power receiving apparatus 4r. With such a system, the wireless power receiving apparatus 4r moves over time. Thus, the degree of coupling between the antennas changes with time. As a result, the conditions for resonance change with time.
In order to provide a supply of electric power over a wide range, an arrangement has been proposed in which a relay device including a resonance circuit is arranged between a power supply apparatus and a power receiving apparatus. In a case in which such a relay device is arranged, this leads to complicated conditions for resonance in the entire system. In order to satisfy such conditions for resonance which change over time, there is a need to provide a variable capacitor to each of the wireless power supply apparatus 2r, the wireless power receiving apparatus 4r, or the relay device, and there is a need to adjust the capacitance of each variable capacitor thus provided so as to satisfy the conditions for resonance. However, in actuality, it is very difficult to detect or estimate the capacitance of each variable capacitor so as to satisfy the conditions for resonance.
In particular, in a case in which multiple relay devices are provided, when the user changes the capacitance of a given variable capacitor, the conditions for resonance also change due to the interaction between the wireless power supply apparatus 2r, the wireless power receiving apparatus 4r, and the multiple relay devices. Thus, in actuality, it is almost impossible to obtain the optimum value of the capacitance to be set for each variable capacitor.
Furthermore, in a case of transmitting a large amount of electric power, the voltage that develops at the resonance circuit has a great amplitude. Thus, the kinds of elements which can be employed as such a variable capacitor are extremely limited from the viewpoint of the breakdown voltage.